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The need to preserve the remains of the Rose Theatre [see figure enlargement 40k], and later those of the Globe Theatre, and to confirm that the site hydrology and water chemistry remained in a state which would ensure the survival of the archaeological deposits, demanded the development of site monitoring procedures and the ongoing monitoring of the burial conditions. Similarly, preservation of the waterlogged Sweet Track within the Shapwick Heath National Nature Reserve had incorporated access tubes to monitor water levels and ensure that the track remained beneath the water table. The monitoring of these sites provided the first information about the nature of physico-chemical conditions in archaeological sites, and the way these conditions change through time.
The EH wetland projects have also increased our understanding of burial environments, and particularly of how land management practices have affected the survival of sites which were formerly in wetlands but have since dried out. Observations at Etton and Flag Fen show that activities which take place at a considerable distance can still have a rapid and significant effect on the burial conditions. At a number of locations, equipment has been installed to monitor the condition of vulnerable sites, and this has enabled us to build on our knowledge to understand better the processes that lead to preservation. At the end of the excavations at Market Deeping a monitoring station was set up straddling the relic water channel in which substantial organic remains survived at or below the water table. The information from this station has enabled us to define the baseline conditions that might be expected in this type of environment and in the soils found within the channel. It has shown how the ground water level fluctuates on an annual cycle and how the moisture content in the soil above the water table also fluctuated. The effects of farming activities were clearly recognised, with extreme changes resulting from the spreading of fertiliser.
The planned extraction of gravel from some 350 hectares at Willingham required careful evaluation of the archaeology, and particularly the effect of quarrying on the two barrow groups within the extraction area. The hydrology of this major archaeological landscape was likely to be changed significantly by the development of the quarry and it was important to assess the effects of draw down of the water levels on any areas having waterlogged preservation. Site monitoring was established as soon as the land became available to determining the hydrological conditions before disturbance commenced. The soil at Willingham is unlike that at Market Deeping: the latter is a fine peaty alluvium which holds water well, and which allows surface water to drain rapidly. At Willingham the soil above the water table is much coarser, the topsoil is heavy clay which holds surface water and does not allow its transmission to the lower levels, the soil moisture profile is therefore very wet at the surface, but has a much lower moisture content below (at about 700 mm), and is wetter again just above the water table.
Soils with a high hydraulic conductivity (water transmission capability) do not retain water, and are unlikely to contain waterlogged archaeological remains unless there is a constant supply of water, or a high water table. Conversely soils such as clays retain water and resist its passage, and have a low hydraulic conductivity which can have the effect of retaining waterlogged conditions in better drained more open soils. Water balance (the relationship between water entering and leaving a soil system) is another important factor in the preservation of waterlogged conditions, a positive water balance (equal input and output) provides good prospects for waterlogged preservation. This is important at the Sweet Track where the water table in the nature reserve is maintained at a higher level than in the surrounding land by pumping and it is important that the volume of water pumped in, equals the volume flowing out of the reserve. Continuous monitoring of water levels through large diameter (100mm) tubes showed that the water table had occasionally dropped below the level of the track. Investigation of the track in 1996 demonstrated that it remained in a good condition, and that when the water table fell below the level of track, there was probably sufficient moisture in the peat to maintain an anoxic environment. This is being tested by an enhanced monitoring programme undertaken by the Royal Holloway Institute for Environmental Research. Water levels and the movement of water through the peat is being assessed, and the degree of oxygen exclusion is being determined across the site. This will allow the management of the site to be refined and will make it possible to determine the extent that English Nature can lower the water table in the summer to permit the harvesting of meadow grass.
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Wetlands are extremely fragile, but other burial environments are also subject to change, most dramatically in the context of urban development. Recently a number of archaeological sites affected by development in the 1960s have been re-examined, and there are concerns that the engineering methods have not been as effective in preventing damage as was expected. In the countryside the impact of change can be no less dramatic and developments in agricultural practice may alter the chemical balances in the soil; the use of nitrates and other chemicals can be expected to change the preservation characteristics of the soil and may allow acceleration of corrosion processes.
The factors influencing the preservation of exposed sites have been investigated at Brading Roman Villa (Isle of Wight), where the Ancient Monuments Laboratory has been advising on the conservation of the high quality mosaics which have deteriorated since their discovery in the nineteenth century and which were seriously affected by flooding in 1994. Together with the CAS, we have undertaken careful evaluation of the condition of the mosaics and their environs, investigated the causes of their deterioration, and applied minimally invasive conservation methods; the work has involved many specialists including geophysicists, environmental scientists, colleagues from the Architectural Conservation Team, Photogrammetric Services, architects, engineers and hydrologists; the conservation has been undertaken by Southern Archaeology. It is hoped that the methods adopted for Brading will offer an alternative to the usual solution for deteriorating mosaics which is to lift them and relay them on a modern foundation; this process destroys the archaeological integrity of mosaics, and often does not remove the underlying causes of deterioration.
Although we are gathering much more information about the nature of the burial environment and the way it interacts with materials buried in it, there are still fundamental questions to be answered. We have little understanding of the short to medium term changes that might occur as a result of changes in soil chemistry, or how this affects the quality of archaeological evidence and our ability to interpret it. In order to promote scientific investigation of the burial environment a proposal has been put to the Natural Environmental Research Council, which is responsible for science based archaeology, for a thematic programme entitled Archaeological Conservation and Environment. If it is approved, the programme will enable major research projects to be undertaken. EH will maintaining its current commitment to the topic, and extend its support to these projects where appropriate.